The present invention relates to an active control system for providing desired force versus displacement characteristics to a pair of manual input devices such as the flight control sticks operated by a pilot and co-pilot flying an aircraft. In addition to providing a desired force versus displacement profile for the input devices, the active control system further acts to reflect manual inputs applied at one of the input devices at the other of the two input devices. Additionally, inputs from an outside source, such as an autopilot signal, can be used to reflect motion onto both control sticks. Variable gain velocity damping is also provided to reduce oscillations at or near a zero or null position.
In many applications it is desirable to impart tactile feedback to users of manually operated input devices. For example, in mechanically linked systems, tactile feedback to the operator is provided as a result of the force required to move the mechanical parts associated with the system. In electronically controlled systems, however, the physical interrelationship between the input device and the mechanical components acted upon is replaced by electrical signals generated by sensors in the input device which signal actuators to act on the mechanical components. In such systems, the force versus displacement characteristics of the input device have no direct relationship to the systems being controlled. It is thus desirable to generate mechanical forces to be applied to the input device to emulate mechanically linked systems. Such emulation provides the operator with tactile feedback regarding the state of the system and the effects of his or her input actions. Heretofore, self-centering control sticks having force versus displacement characteristics that emulate mechanical systems have employed mechanical spring arrangements or active servo control systems.
Aircraft flight control systems are an application where it is particularly important to provide accurate tactile feedback to the pilot or co-pilot operating a control stick or yoke which electrically interfaces with the mechanical systems for controlling the flight control surfaces of the aircraft. Such xe2x80x9cfly by wirexe2x80x9d systems employ various sensors to determine the position of and/or force applied to the control stick in order to translate the pilot""s input commands into electrical signals for controlling the flight control surfaces of the aircraft. In many aircraft, dual control sticks are provided, one to be operated by the pilot and a second to be operated by a co-pilot. In cases where there are dual control sticks, it is desirable that actions taken on either one of the control sticks are reflected in the other control stick in the form of a force supplied to the second control stick in the direction of the action taken on the first control stick.
An active control system for providing variable force feel characteristics to a pair of manual input control stick is disclosed in U.S. Pat. No. 5,291,113 to Hegg et al. According to the system disclosed there, a desired force versus displacement profile is provided in which the magnitude of the control stick displacement is proportional to the force applied in order to emulate a purely mechanically linked system. The system includes a pair of control sticks each of which is directly coupled through a gimble to a motor in a conventional manner. A position signal from the first input device is fed back and combined with an autopilot or center position signal to create an error signal. This error signal is amplified and input to servo control electronics to generate excitation currents for a motor coupled to the control stick. Thus, this position feedback loop causes the motor to drive the control stick in a direction to reduce the amount of error between the position commanded by the autopilot signal and the actual position of the control stick. The gain of the amplifier that acts on the difference between the stick position and the autopilot reference command in Hegg, et al. is fixed, and serves to define the mechanical spring rate being emulated, resulting in a single force-versus-displacement gradient profile.
A second position signal generated from the position of the second control stick is also fed back and summed with the position signal from the first control stick. This signal is also amplified and summed with the error signal input to the servo controller driving the motor coupled to the first control stick. The signal representing the combined position signal from the first and second control sticks is amplified to a far greater extent than the error signal between the autopilot signal in the position of the first control stick. Thus, the position error signal between the first and second control sticks will dominate over the position error signal between the autopilot signal and the first control sticks. The motor coupled to the first control stick will drive the first control stick to a position intended to eliminate the position error between the first and second control sticks, as well as attempting to reconcile the position between the first control stick and the position commanded by the autopilot signal, with elimination of the position error between the two control sticks predominating.
The position signals from both the first and second control sticks are also in fed into the servo control electronics driving the motor coupled to the second control stick. Thus, displacement of the first control stick will also be reflected back to the second control stick. Discrepancies between the autopilot signal and the second control stick are rectified by having the second control stick follow the position of the first control stick. The operational characteristics of such a system are poor since the system relies on the reconciled position of one stick as the signal to drive the other. This can cause poor frequency response, lag in position tracking, poor coupling and poor feel.
A second embodiment disclosed by Hegg et al. further describes a torque sensor for generating a signal representative of the torque applied to the first and second control sticks. These signals are fed back and summed with the position error signals which are input to the servo control electronics driving the motors which are coupled to the first and second control sticks.
Another example of an active control system for controlling the force feel characteristics of a manual input device such as a flight control stick is disclosed in U.S. Pat. No. 5,347,204 Gregory et al. A system is disclosed there for providing variable damping to a servo control system in order to prevent oscillations due to motor torque and high gain characteristics at or near the center position. A signal representing the angular velocity of the control stick is combined with the position error signal which is supplied to the servo control electronics driving the motor coupled to the control stick. The velocity feedback signal is subjected to position dependent scaling which provides a variable rate gain which is dependent on the angular position of the control stick. The position dependent scaling is implemented via an amplifier inserted in the velocity feedback loop. The gain of the amplifier is established by a pair of resistors connected in parallel between one of the inputs and the output of the amplifier. A position dependent switch is connected in series with one of the resistors such that when the control sticks is positioned within a first position range the switch is open, and the gain of the amplifier is determined by only one of the resistors connected across the input and output of the amplifier. When the control stick is in a second position range, the switch is closed and the gain of the amplifier is determined by the parallel combination of the two resistors. Thus, a higher gain setting for the feedback amplifier may be established when the control stick is near the zero position to provide higher rate damping for the overall servo loop when the control stick is near the zero position, and less rate damping as the control stick is moved away from zero in order to improve the response characteristics of the system.
The present invention provides significant advantages over prior art active control systems for dual input control devices. The active control system of the present invention provides for multi-shaped force versus displacement profiles in a simpler, less expensive manner than the prior art. The present system provides excellent frequency response with little or no lag in position tracking, strong coupling between the input devices, with whatever tactile response is desired. All of these features are provided without the added cost and complexity of single or redundant multiple force or torque sensors. Thus, the system is less expensive and more reliable than prior art active control systems.
The present invention relates to an active control system for tactile feedback to an operator employing a manual input device such as a flight control stick used for flying an aircraft. Specifically, the invention provides desired force versus displacement characteristics to each of a pair of input devices, such as the pilot""s and co-pilot""s flight control sticks. In addition to supplying a centering force to urge the input devices back toward a predefined center position when the input devices are manually displaced, the active control system will also act to reconcile the positions of the two input devices with a command signal received from an external source such as an autopilot. Finally, differences in position between the first and second input devices are reflected back to each other by way of a restoring torque which tends to force each input device in the direction of the position of the other. Thus, if one operator, such as a pilot moves his or her flight control stick, the pilot""s action will be reflected as a force applied to the co-pilot""s control stick in the direction in which the position of the pilot""s control stick varies from the position of the co-pilot""s control stick.
Each input device is configured to receive a manual torque input for angularly displacing the input device about a control axis. Each input device may include more than one control axis. For instance, a single flight control stick may be configured to receive manual input for controlling the pitch, roll, and yaw of an aircraft, by moving the control stick relative to three separate control axes. In cases where the input device comprises multiple control axes, the active control system of the present invention may be duplicated on each axis. However, for the sake of brevity and clarity, the system is described herein as applied to only a single control axis of each input device.
A servo motor is coupled to each of the first and second input devices in a manner whereby the motor can apply torque about the control axes of the two input devices. As will be described below, the torque applied by the motors will generally be a restorative torque directed toward returning the input devices to a center or null position, or toward reconciling the position of the input devices with a position command signal supplied by an external system such as another side stick or the autopilot system.
First and second servo control loops are associated with the first and second motors respectively. Each servo control loop comprises a position sensor for generating a position signal indicative of the angular position of the corresponding input device. A force profile gain amplifier and a servo controller are also included in the first and second servo control loops. The force profile gain amplifier receives a position error signal derived by subtracting the angular position signal output by the position sensor from the command signal received from an external system. If the external system is not operable or not present, the position error signal merely becomes the negative of the position signal.
The gain of the force profile amplifier is variable, and is a function of the angular position of the input device which is fed back to the force profile gain amplifier from the position sensor. This technique allows shaping of the force profile to include multiple segments of different shapes, not limited to, but including: breakout regions, main gradient, soft stop, post-soft-stop gradient, and hard stop. All of these features can be modifiable in real-time in terms of their magnitude and position.
A torque error signal is output from the force profile amplifier and input to the servo controller. The servo controller converts the torque error signal to current for driving the motor. The polarity and magnitude of the motor drive current are such as to drive the motor in a direction opposite the direction of displacement of the associated input device, with a restorative torque proportional to the amount of displacement.
A cross-coupling feedback loop is also provided for reflecting the relative positions of the first and second input devices in the torque error signal input to the servo controllers associated with the first and second servo control loops. This has the effect of altering the torque applied to the first and second input devices to reflect a force directed toward reconciling the positions of the first and second input devices.
Each of the first and second servo control loops includes a velocity damping loop. The amount of velocity damping is dependent on the velocity of the associated input device. An angular velocity signal proportional to the angular velocity of the input device is fed back to a velocity damping profile amplifier both as an input signal, and as the signal to be amplified. The variable gain of the damping profile increases with the velocity of the respective input device. Thus, the faster the input device is moved, the greater the damping applied. The output of the profile damping amplifier is subtracted from the torque error signal and acts to smooth the torque characteristics applied to the input device.
The cross-coupling feedback loop includes a relative position signal representing the difference between the angular position signal of the second input device and the angular position signal of the first input device. A proportional gain amplifier amplifies the relative position signal, and an integrating amplifier integrates and amplifies the relative position signal. Also, a relative velocity signal is included in the cross-coupled feedback loop. The relative velocity signal is obtained by taking the difference between the first input device angular velocity signal and the second input device angular velocity signal. A signal summing device adds the integrated relative position signal output from the integrating amplifier to the proportionally amplified relative position signal output from the proportional gain amplifier and subtracts the relative velocity signal. The resultant cross-coupled position signal is input to a cross-coupled damping amplifier, the output of which is added to the torque error signal of one of the first and second servo control loops and subtracted from the other of the first and second servo control loops.